SCF (SKP1-Cullins-F box protein) E3 ubiquitin ligase is the largest family of E3 ligases that promote the ubiquitylation and degradation of various key regulatory proteins, thus controlling many important biological processes, such as apoptosis, cell cycle progression, DNA replication, embryogenesis and tumorigenesis. SCF consists of 4 components, including a scaffold protein cullin-1, an adaptor protein SKP1, a RING protein RBX1 or RBX2 and an F-box protein. The F-box protein with 69 members in mammalian cells is the substrate- recognizing subunit that determines the specificity of SCF E3s. While a variety of protein substrates have been identified by one of well-characterized F-box proteins TrCP, FBXW7, or SKP2, very little is known as to whether and how F-box proteins regulate each other. Our preliminary data showed that 1) oncogenic F-box protein ?TrCP binds to yet a poorly characterized F-box protein FBXW2 to promote its ubiquitylation and degradation; 2) FBXW2 in turn binds to oncogenic F-box protein SKP2 to promote its ubiquitylation and degradation; 3) FBXW2 has tumor suppressive function against lung cancer cells; 4) A number of gain-of- function FBXW2 mutations were found in human non-small cell lung carcinoma tissues; and 5) reversed correlation in expression among ?-TrCP, FBXW2 and SKP2 exists in lung cancer tissues, which is associated with patient survival. Although ?-TrCP and SKP2 are well-studied F-box proteins with oncogenic activity, whether and how they regulate each other with regard to cell proliferation and survival is totally unknown. The objectives of this proposed study is to understand how FBXW2 mediates and modulates the activity of two well-known oncogenic F-box proteins, ?-TrCP and SKP2, and how FBXW2 acts as a novel tumor suppressor in lung tumorigenesis using both in vitro cell culture and in vivo genetically modified mouse models with elucidation of its mechanism of action. The central hypothesis is that three F- box proteins, ?TrCP-FBXW2-SKP2 form an oncogene-tumor suppressor-oncogene axis that regulates each other to control cell proliferation and lung tumorigenesis. Three specific aims are proposed to test our central hypothesis by 1) characterizing FBXW2 as a novel substrate of ?-TRCP; 2) characterizing FBXW2 as a novel E3 ligase for SKP2 degradation; and 3) characterizing Fbxw2 as a novel tumor suppressor against lung tumorigenesis. Relevance: Successful completion of this proposed study will mechanistically elucidate how three F-box proteins regulate proliferation of lung cancer cells by targeting each other for degradation, and how two oncogenic F-box proteins -TrCP and SKP2 talk with each other via a tumor suppressive F-box protein FBXW2, and how FBXW2 acts as a tumor suppressor in the lung, thus elucidating a novel mechanism of lung tumorigenesis and providing attractive targets for future biomarker development in lung cancer management.